Radioactive (minus) erythrofluorocitrate is being used as a molecular probe to study macromolecular components of the mitochondrial citrate transport system. Two enzymes, both specific for citrate were labelled covalently by fluorocitrate, forming protein thiolesters. Both enzymes are identified as glutathione citryl thioester synthetase and hydrolase, both inhibited by fluorocitrate. Equivalent thioester enzymes are identified for all carboxylic acid substrates of the Krebs cycle. The new enzymes are being characterized and isolated and their physiological significance determined.